External resonance-type laser module

ABSTRACT

An external resonance-type laser module includes: a quantum cascade laser; a MEMS diffraction grating including a movable portion capable of swinging around an axis and a diffraction grating portion formed on the movable portion; and a lens. The diffraction grating portion includes a plurality of lattice grooves arranged in a first direction and each of the plurality of lattice grooves extends in a second direction perpendicular to the first direction. The MEMS diffraction grating is disposed such that a normal line of the diffraction grating portion is inclined with respect to an end surface and the first direction is along a lamination direction of a laminated structure when viewed in a direction perpendicular to the end surface. A length of the diffraction grating portion in the first direction exceeds a length of the diffraction grating portion in the second direction.

TECHNICAL FIELD

One aspect of the present disclosure relates to an externalresonance-type laser module provided with a micro electro mechanicalsystems (MEMS) diffraction grating.

BACKGROUND

An external resonance-type laser module including a quantum cascadelaser, a swingable diffraction grating, and a lens disposed between thequantum cascade laser and the diffraction grating is known (see, forexample, Japanese Unexamined Patent Publication No. 2019-036577). Insuch an external resonance-type laser module, light from the quantumcascade laser is diffracted and reflected by the diffraction grating andlight having a specific wavelength in the light is returned to thequantum cascade laser. As a result, an external resonator is configuredby an end surface of the quantum cascade laser and the diffractiongrating and light having a specific wavelength is amplified and outputto the outside.

A reduction in power consumption is required in the externalresonance-type laser module as described above. In particular, when thequantum cascade laser is used, the beam diameter of the light emittedfrom the quantum cascade laser is large, and thus it is necessary toincrease the area of the diffraction grating. Accordingly, a largedriving force is required and an increase in power consumption is likelyto occur. In addition, a reduction in spectral line width is requiredfor an increase in output light purity and the external resonance-typelaser module is required to be reduced in size.

SUMMARY

In this regard, an object of one aspect of the present disclosure is toprovide an external resonance-type laser module that enables sizereduction, power consumption reduction, and spectral line widthreduction.

An external resonance-type laser module according to one aspect of thepresent disclosure includes: a quantum cascade laser including alaminated structure and emitting light from an end surface; a MEMSdiffraction grating including a movable portion capable of swingingaround a predetermined axis and a diffraction grating portion formed onthe movable portion, in which the MEMS diffraction grating diffracts andreflects the light emitted from the quantum cascade laser by thediffraction grating portion, and returns a part of the light to thequantum cascade laser; and a lens disposed between the quantum cascadelaser and the MEMS diffraction grating, in which the diffraction gratingportion includes a plurality of lattice grooves arranged in a firstdirection and each of the plurality of lattice grooves extends in asecond direction perpendicular to the first direction, the MEMSdiffraction grating is disposed such that a normal line of thediffraction grating portion is inclined with respect to the end surfaceand the first direction is along a lamination direction of the laminatedstructure when viewed in a direction perpendicular to the end surface,and a length of the diffraction grating portion in the first directionexceeds a length of the diffraction grating portion in the seconddirection.

In this external resonance-type laser module, the MEMS diffractiongrating is disposed such that the normal line of the diffraction gratingportion is inclined with respect to the end surface of the quantumcascade laser and the length of the diffraction grating portion in thefirst direction, which is the arrangement direction of the latticegrooves, exceeds the length of the diffraction grating portion in thesecond direction perpendicular to the first direction. Since thediffraction grating portion is long in the first direction, the lightfrom the quantum cascade laser can be satisfactorily received by thediffraction grating portion even when the diffraction grating portion isdisposed in an inclined matter. In addition, since the length of thediffraction grating portion in the second direction is reduced, themodule can be reduced in size and it is possible to suppress an increasein power consumption by suppressing an increase in the size of thediffraction grating portion. Further, in this external resonance-typelaser module, the MEMS diffraction grating is disposed such that thefirst direction is along the lamination direction of the laminatedstructure when viewed in the direction perpendicular to the end surfaceof the quantum cascade laser. By disposing the MEMS diffraction gratingsuch that the first direction is along the lamination direction, theextension direction of the lattice groove in the diffraction gratingportion (second direction) can be orthogonal to the polarizationdirection of the light emitted from the quantum cascade laser. Inaddition, by the first direction, which is the arrangement direction ofthe lattice grooves, being the longitudinal direction of the MEMSdiffraction grating, the lattice grooves positioned within theirradiation beam diameter in the inclined diffraction grating portioncan be increased in number. As a result, a high wavelength resolution ofthe MEMS diffraction grating can be ensured and the spectral line widthof the output light can be reduced. Accordingly, with this externalresonance-type laser module, the size, power consumption, and spectralline width can be reduced.

The movable portion may be formed in a shape having four corner portionswhen viewed in a direction parallel to the normal line, and the fourcorner portions may be formed in a round shape. In this case, the momentof inertia of the movable portion can be reduced and the swing of themovable portion can be increased in speed.

The light emitted from the quantum cascade laser and incident on theMEMS diffraction grating via the lens may have a beam shape in which alength in the first direction exceeds a length in the second directionat a position of the diffraction grating portion. Since the length ofthe diffraction grating portion in the first direction exceeds thelength of the diffraction grating portion in the second direction, thelight from the quantum cascade laser can be satisfactorily received bythe diffraction grating portion even in such a case.

The MEMS diffraction grating may be disposed such that all of the lightemitted from the quantum cascade laser and transmitted through the lensis incident on the diffraction grating portion regardless of a swingangle of the movable portion around the axis. In this case, it ispossible to suppress a situation in which a part of the light from thequantum cascade laser becomes stray light without entering thediffraction grating portion.

According to one aspect of the present disclosure, it is possible toprovide an external resonance-type laser module that enables sizereduction, power consumption reduction, and spectral line widthreduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser module according to anembodiment.

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1.

FIG. 3 is a perspective view of a diffraction grating unit.

FIG. 4 is a view of a MEMS diffraction grating viewed in a normaldirection.

FIG. 5 is a view of the MEMS diffraction grating viewed in the normaldirection.

FIG. 6 is a cross-sectional view of a diffraction grating portion takenalong line VI-VI of FIG. 5.

FIG. 7 is a perspective view of a magnet.

FIG. 8A is a plan view of the magnet, and FIG. 8B is a side view of themagnet.

FIGS. 9A and 9B are plan views of MEMS diffraction gratings of first andsecond modification examples.

DETAILED DESCRIPTION

Hereinafter, one embodiment of the present disclosure will be describedin detail with reference to the drawings. In the following description,the same reference numerals are used for the same or equivalent elementswith redundant description omitted. The X, Y, and Z directions in thedrawings are set for convenience.

As illustrated in FIGS. 1 and 2, an external resonance-type laser module1 (hereinafter, referred to as “laser module 1”) includes a housing 2, amount member 3, a quantum cascade laser 4 (hereinafter, referred to as“QCL 4”), a diffraction grating unit 5, and lenses 6 and 7. The mountmember 3, the QCL 4, the diffraction grating unit 5, and the lenses 6and 7 are accommodated in the housing 2. The housing 2 constitutes, forexample, a butterfly package. As an example, the length of each side ofthe housing 2 is 30 mm or less. The housing 2 has a main body portion 21having an opening 21 a and a lid portion 22 blocking the opening 21 aand is formed in a box shape. The housing 2 has an emission window 2 afor outputting output light L of the laser module 1 to the outside.

The mount member 3 is fixed to the bottom surface of the housing 2. TheQCL 4, the diffraction grating unit 5, and the lenses 6 and 7 are fixedto the mount member 3. More specifically, the mount member 3 has a mainbody portion 31 and a protruding portion 32 protruding in the Zdirection from the main body portion 31. The QCL 4 is fixed to the topsurface of the protruding portion 32. A disposition hole 33 is formed inthe main body portion 31. The diffraction grating unit 5 is fixed to themain body portion 31 in a state where a part of a yoke 53, which will bedescribed later, is disposed in the disposition hole 33. The lenses 6and 7 are held by lens holders 11 and 12, respectively. The lens 6 isfixed to the main body portion 31 via the lens holder 11 and is disposedbetween the QCL 4 and a MEMS diffraction grating 51 of the diffractiongrating unit 5. The lens 7 is fixed to the main body portion 31 via thelens holder 12 and is disposed between the QCL 4 and the emission window2 a. The emission window 2 a, the lens 7, the QCL 4, the lens 6, and theMEMS diffraction grating 51 are arranged in this order in the Ydirection.

The QCL 4 has a first end surface 4 a and a second end surface 4 b onthe side opposite to the first end surface 4 a and emits light in amid-infrared region (for example, 4 μm to 12 μm) from each of the firstend surface 4 a and the second end surface 4 b. The first end surface 4a and the second end surface 4 b are, for example, flat surfacesperpendicular to the Y direction. The first end surface 4 a isantireflection-coated.

The QCL 4 has a semiconductor substrate 41 and a laminated structure 42formed on the semiconductor substrate 41. The laminated structure 42includes an active layer and a pair of clad layers sandwiching theactive layer. The active layer includes, for example, a plurality ofquantum well layers made of InGaAs and a plurality of quantum barrierlayers made of InAlAs. The clad layer is made of, for example, InP. Theactive layer and the clad layer are formed on the semiconductorsubstrate 41 by crystal growth. During the crystal growth, the activelayer and the clad layer are formed on the semiconductor substrate 41along the Z direction (lamination direction, growth direction). Thegrowth direction is the thickness direction of the active layer. Thelight emitted from the QCL 4 is linearly polarized light parallel to thelamination direction. The laminated structure 42 may include a pair ofclad layers and a plurality of active layers having different centerwavelengths.

The lenses 6 and 7 are, for example, aspherical lenses made of zincselenide (ZnSe). The surfaces of the lenses 6 and 7 areantireflection-coated. The lens 6 is disposed on the side of the firstend surface 4 a with respect to the QCL 4 and collimates the lightemitted from the first end surface 4 a. The lens 7 is disposed on theside of the second end surface 4 b with respect to the QCL 4 andcollimates the light emitted from the second end surface 4 b. The lightcollimated by the lens 7 passes through the emission window 2 a of thehousing 2 and is output to the outside as the output light L.

The light collimated by the lens 6 is incident on the MEMS diffractiongrating 51 of the diffraction grating unit 5. By diffracting andreflecting this incident light, the MEMS diffraction grating 51 returnsthe light that has a specific wavelength in the incident light to thefirst end surface 4 a of the QCL 4. In the laser module 1, the MEMSdiffraction grating 51 and the second end surface 4 b constitute aLittrow-type external resonator. As a result, the laser module 1 iscapable of amplifying light having a specific wavelength and outputtingthe light to the outside.

In addition, in the MEMS diffraction grating 51, the direction of adiffraction grating portion 64 diffracting and reflecting incident lightcan be changed at a high speed as will be described later. As a result,the wavelength of the light returning from the MEMS diffraction grating51 to the first end surface 4 a of the QCL 4 is variable and, andfurthermore, the wavelength of the output light L of the laser module 1is variable. By changing the wavelength of the output light L,wavelength sweeping can be performed within, for example, the range ofthe gain band of the QCL 4.

The diffraction grating unit 5 includes the MEMS diffraction grating 51,a magnet 52, and the yoke 53. The MEMS diffraction grating 51 is formedin a substantially plate shape. The magnet 52 is disposed on the sideopposite to the QCL 4 with respect to the MEMS diffraction grating 51.The MEMS diffraction grating 51 is fixed to the yoke 53, and the magnet52 is accommodated in the yoke 53. As a result, the MEMS diffractiongrating 51, the magnet 52, and the yoke 53 are integrated and constituteone unit.

As illustrated in FIGS. 3 to 6, the MEMS diffraction grating 51 includesa support portion 61, a pair of connecting portions 62, a movableportion 63, the diffraction grating portion 64, and a pair of coils 65and 66. The MEMS diffraction grating 51 is configured as a MEMS deviceswinging the movable portion 63 around an axis A. The MEMS diffractiongrating 51 is formed by processing a semiconductor substrate using aMEMS technique (such as patterning and etching).

The support portion 61 is a flat plate-shaped frame body having arectangular shape in a plan view (when viewed in a normal direction DNto be described later). The support portion 61 supports the movableportion 63 via the pair of connecting portions 62. Each of theconnecting portions 62 is a flat plate-shaped member having arectangular rod shape in a plan view and extends straight along the axisA. Each of the connecting portions 62 connects the movable portion 63 tothe support portion 61 on the axis A such that the movable portion 63 iscapable of swinging around the axis A.

The movable portion 63 is positioned inside the support portion 61. Asdescribed above, the movable portion 63 is capable of swinging aroundthe axis A. The movable portion 63 is a flat plate-shaped member havinga substantially rectangular shape in a plan view and has four cornerportions 63 a. The movable portion 63 is chamfered in an R shape in eachof the corner portions 63 a, and each of the corner portions 63 a isformed in a round shape. For example, each of the corner portions 63 ais curved in a circular arc shape in a plan view. As a result, themoment of inertia of the movable portion 63 can be reduced and the swingof the movable portion 63 can be increased in speed. In this example,the movable portion 63 is formed in a substantially rectangular shape inwhich the long side is parallel to a first direction D1 and the lengthof the movable portion 63 in the first direction D1 exceeds the lengthof the movable portion 63 in a second direction D2. As an example, thelength of the movable portion 63 in the first direction D1 isapproximately 4 mm, the length of the movable portion 63 in the seconddirection D2 is approximately 3 mm, and the thickness of the movableportion 63 is approximately 30 μm. The support portion 61, theconnecting portion 62, and the movable portion 63 are integrally formedby, for example, being built into one silicon-on-insulator (SOI)substrate.

The diffraction grating portion 64 is provided on the surface of themovable portion 63 on the side of the QCL 4. The diffraction gratingportion 64 has a plurality of lattice grooves 64 a and diffracts andreflects the light emitted from the QCL 4. The diffraction gratingportion 64 includes, for example, a resin layer provided on the surfaceof the movable portion 63 and having a diffraction grating pattern and ametal layer provided over the surface of the resin layer so as to followthe diffraction grating pattern. Alternatively, the diffraction gratingportion 64 may be configured only by a metal layer provided on themovable portion 63 and having a diffraction grating pattern. Althoughthe diffraction grating pattern in this example is a blazed grating witha serrated cross section for an increase in diffraction efficiency, thepattern may be a binary grating with a rectangular cross section, aholographic grating with a sinusoidal cross section, or the like. Thediffraction grating pattern is formed on the resin layer by, forexample, a nanoimprint lithography method. The metal layer is, forexample, a metal reflective film made of gold and is formed byevaporation.

As illustrated in FIG. 5, the plurality of lattice grooves 64 a arearranged at equal intervals in the first direction D1. Each of thelattice grooves 64 a extends straight in the second direction D2perpendicular to the first direction D1. The second direction D2 isparallel to the axis A. A repeat cycle (distance between the adjacentlattice grooves 64 a) d of the lattice grooves 64 a in the firstdirection D1 is, for example, 4 μm to 10 μm. An angle (blazed angle) θof the lattice groove 64 a with respect to the normal direction DNparallel to a normal line (straight line perpendicular to a latticesurface S) N of the diffraction grating portion 64 is, for example, 20degrees to 35 degrees.

The diffraction grating portion 64 is formed to be one size smaller thanthe movable portion 63 in a plan view, and the outer edge of thediffraction grating portion 64 extends along the outer edge of themovable portion 63 at a certain interval from the outer edge of themovable portion 63. In this example, the diffraction grating portion 64is formed in a substantially rectangular shape similar to the movableportion 63 in a plan view. In other words, the diffraction gratingportion 64 is formed in a substantially rectangular shape in which thelong side is parallel to the first direction D1 and a length L1 of thediffraction grating portion 64 in the first direction D1 exceeds alength L2 of the diffraction grating portion 64 in the second directionD2.

The coils 65 and 66 are made of a metal material such as copper and havea damascene structure embedded in the groove formed in the surface ofthe movable portion 63. In a plan view, the coil 65 is disposed on oneside (upper side in FIG. 4) with respect to the axis A and the coil 66is disposed on the other side (lower side in FIG. 4) with respect to theaxis A.

Each of the coils 65 and 66 is spirally wound a plurality of times in aplan view. The outside end portion of the coil 65 is electricallyconnected via wiring 72 to an electrode pad 71 provided on the supportportion 61. The wiring 72 extends over the support portion 61, oneconnecting portion 62, and the movable portion 63. The outside endportion of the coil 66 is electrically connected via wiring 74 to anelectrode pad 73 provided on the support portion 61. The wiring 74extends over the support portion 61, the other connecting portion 62,and the movable portion 63.

The inside end portion of the coil 65 is electrically connected to theinside end portion of the coil 66. In this example, the inside endportions of the coils 65 and 66 are electrically connected to each otherby multilayer wiring 67. The multilayer wiring 67 can also be regardedas constituting a part of the coils 65 and 66, and it is alsoconceivable that the pair of coils 65 and 66 are configured by one coilwiring (multilayer wiring) extending so as to be folded back in an eight(8) shape in a plan view. The coils 65 and 66 may be formed integrallywith each other.

The coil 65 has inside parts 65 a extending along the axis A and outsideparts 65 b extending on the outer edge side of the movable portion 63.The inside parts 65 a and the outside parts 65 b extend in a straightline and in parallel to each other along the second direction D2. Theinside parts 65 a extend along the axis A at a certain interval from theaxis A. The outside parts 65 b extend along the outer edge of themovable portion 63 at a certain interval from the outer edge of themovable portion 63.

The coil 66 has inside parts 66 a extending along the axis A and outsideparts 66 b extending on the outer edge side of the movable portion 63.The inside parts 66 a and the outside parts 66 b extend in a straightline and in parallel to each other along the second direction D2. Theinside parts 66 a extend along the axis A at a certain interval from theaxis A. The outside parts 66 b extend along the outer edge of themovable portion 63 at a certain interval from the outer edge of themovable portion 63.

The magnet 52 generates a magnetic field (magnetic force) acting on thecoils 65 and 66. As illustrated in FIGS. 2, 7, 8A, and 8B, the magnet 52is a neodymium magnet (permanent magnet) formed in a substantiallyrectangular parallelepiped shape and has a surface 54. The surface 54 isa surface on the side of the MEMS diffraction grating 51 and faces theMEMS diffraction grating 51 in the normal direction DN. The magnet 52 ismade of a single member having different magnetic poles on one side andthe other side in the normal direction DN. In other words, the magnet 52is not a magnet configured by combining a plurality of members but anintegrated bulk magnet magnetized in the thickness direction. In thisexample, the magnet 52 has an N pole part 57 on the side of the MEMSdiffraction grating 51 and an S pole part 58 on the side opposite to theMEMS diffraction grating 51. In FIGS. 8A and 8B, the boundary linebetween the N pole part 57 and the S pole part 58 is indicated by atwo-dot chain line.

The surface 54 includes a pair of inclined surfaces 55. Each of theinclined surfaces 55 is inclined such that the distance from the movableportion 63 of the MEMS diffraction grating 51 increases as the distancefrom the axis A increases when viewed in the second direction D2. Theinclination angle of the inclined surface 55 (angle with respect to aplane perpendicular to the normal direction DN) is set to half of themaximum mechanical inclination angle of the movable portion 63 in theMEMS diffraction grating 51. As a result, it is possible to suppress themovable portion 63 interfering with the magnet 52 when the movableportion 63 swings around the axis A and the swing angle of the movableportion 63 can be increased. For example, when the maximum mechanicalinclination angle is 10 degrees, the inclination angle of the inclinedsurface 55 is set to 5 degrees. As illustrated in FIG. 8B, when viewedin the second direction D2, straight lines that are virtual extensionsof the pair of inclined surfaces 55 intersect at an intersection B on astraight line parallel to the normal direction DN through the axis A.

A recess portion 56 is formed in the surface 54. In this example, therecess portion 56 is a groove extending straight along the seconddirection D2. The recess portion 56 has, for example, a rectangularcross section uniform with respect to the second direction D2 and has aflat bottom surface 56 a perpendicular to the normal direction DN. Inthe first direction D1, the recess portion 56 is disposed between thepair of inclined surfaces 55 and is connected to the pair of inclinedsurfaces 55. In the second direction D2, the recess portion 56 extendsso as to reach both end portions of the magnet 52. The space in therecess portion 56 is a void. Alternatively, a non-magnetic body may bedisposed in the space (the space may be filled with a non-magneticbody). The recess portion 56 is formed at the N pole part 57 so as toremain within the N pole part 57 and does not reach the S pole part 58.This is because an increase in the complexity of the line of magneticforce may arise and stable operation may be hindered if the recessportion 56 is formed so as to reach the S pole part 58 or the recessportion 56 is a through hole.

The positional relationship between the MEMS diffraction grating 51 andthe magnet 52 that are viewed in the normal direction DN will bedescribed with reference to FIG. 4. The length of the MEMS diffractiongrating 51 in the first direction D1 exceeds the length of the magnet 52in the first direction D1. The length of the magnet 52 in the firstdirection D1 is substantially equal to the length of the diffractiongrating portion 64 in the first direction D1. The length of the MEMSdiffraction grating 51 in the second direction D2 is substantially equalto the length of the magnet 52 in the second direction D2. The length ofthe magnet 52 in the second direction D2 exceeds the length of thediffraction grating portion 64 in the second direction D2. With such apositional relationship, it is possible to avoid the light from the QCL4 being incident on the magnet 52 when the movable portion 63 swingsaround the axis A while making the magnet 52 as large as possible.

At least a part of the recess portion 56 (bottom surface 56 a) overlapsthe inside parts 65 a and 66 a of the coils 65 and 66 when viewed in thenormal direction DN. In this example, a part of the recess portion 56 onthe side of the axis A overlaps the entire inside parts 65 a and 66 a.In other words, when viewed in the normal direction DN, a width W1 ofthe recess portion 56 exceeds a width W2 of the inside parts 65 a and 66a. The widths W1 and W2 are widths in the first direction D1. The recessportion 56 is positioned closer to the side of the axis A than theoutside parts 65 b and 66 b of the coils 65 and 66 when viewed in thenormal direction DN and does not overlap the outside parts 65 b and 66b. As an example, the length of the magnet 52 in the first direction D1is 4 mm and the length of the magnet 52 in the second direction D2 is 6mm The width W1 of the recess portion 56 is approximately 1 mm to 2 mmand is, for example, 2 mm The depth of the recess portion 56 (distancebetween the bottom surface 56 a and the intersection B) is approximately1 mm, and the thickness of the magnet 52 in the normal direction DN is 3mm to 3.5 mm.

The yoke 53 amplifies the magnetic force of the magnet 52 and forms amagnetic circuit together with the magnet 52. As illustrated in FIG. 2,the yoke 53 has an inclined surface 53 a. The inclined surface 53 aextends flat and perpendicularly to the normal direction DN and isinclined with respect to the first end surface 4 a of the QCL 4. Byfixing the MEMS diffraction grating 51 on the inclined surface 53 a, anormal line N of the diffraction grating portion 64 of the MEMSdiffraction grating 51 can be inclined with respect to the first endsurface 4 a. In this example, the diffraction grating portion 64 isinclined so as to face one side in the Z direction (side of the lidportion 22 of the housing 2). Alternatively, the diffraction gratingportion 64 may be inclined so as to face the other side in the Zdirection (bottom surface side of the housing 2). The inclination angleof the inclined surface 53 a (angle with respect to the first endsurface 4 a) is set in accordance with the oscillation wavelength of theQCL 4, the number of grooves of the lattice grooves 64 a in thediffraction grating portion 64, and the angle θ. For example, when theoscillation wavelength is approximately 7 μm and the number of groovesis 150 per millimeter, the inclination angle of the inclined surface 53a is set to approximately 30 degrees.

The yoke 53 is formed in a substantially U shape (inverted C shape) whenviewed in the X direction and defines a disposition space SP open to theinclined surface 53 a. The magnet 52 is disposed in the dispositionspace SP, and the magnet 52 is accommodated in the yoke 53. The yoke 53surrounds the magnet 52 when viewed in the X direction. The MEMSdiffraction grating 51 is fixed to the inclined surface 53 a in the edgeportion of the support portion 61 so as to cover the opening of thedisposition space SP. The MEMS diffraction grating 51 is disposed suchthat the first direction D1 is along (parallel to) the Z direction(lamination direction of the laminated structure 42 of the QCL 4) andthe second direction D2 (axis A) is parallel to the X direction whenviewed in the Y direction.

The QCL 4 emits light having a beam shape that is long in the laminationdirection of the laminated structure 42. In the laser module 1, thelight emitted from the QCL 4 and incident on the MEMS diffractiongrating 51 via the lens 6 has an elliptical beam shape in which thelength in the first direction D1 exceeds the length in the seconddirection D2 at the position of the diffraction grating portion 64. Inaddition, the MEMS diffraction grating 51 is disposed such that all ofthe light emitted from the QCL 4 and transmitted through the lens 6 isincident on the diffraction grating portion 64 regardless of the swingangle of the movable portion 63 around the axis A. In other words, theentire light is incident on the diffraction grating portion 64 even in astate where the movable portion 63 swings around the axis A up to themaximum mechanical inclination angle.

As illustrated in FIG. 3, the magnet 52 and the yoke 53 form a magneticfield M passing through the MEMS diffraction grating 51. In thisexample, the magnetic field M is formed so as to cross the movableportion 63 from the side of the axis A toward the edge portion side ofthe movable portion 63. The yoke 53 is formed of a carbon-added ironmaterial. As a result, a processing process for the inclined surface 53a can be facilitated. The surface of the yoke 53 may be protected bygalvanization or the like for deterioration prevention.

When currents flow through the coils 65 and 66 in the MEMS diffractiongrating 51, the magnetic field M formed by the magnet 52 and the yoke 53causes the Lorentz force in a predetermined direction in the electronsflowing in the coils 65 and 66. As a result, the coil 65 receives aforce in a predetermined direction. Accordingly, the movable portion 63(diffraction grating portion 64) can be swung around the axis A bycontrolling, for example, the direction or magnitude of the currentflowing through the coil 65. In addition, the movable portion 63 can beswung at a high speed at a resonance frequency level (for example, at afrequency of 1 kHz or more) by passing currents having a frequencycorresponding to the resonance frequency of the movable portion 63through the coils 65 and 66. In the manner, the coils 65 and 66, themagnet 52, and the yoke 53 function as actuator portions swinging themovable portion 63.

In FIG. 4, the directions of the currents flowing through the coils 65and 66 are indicated by arrows. As illustrated in FIG. 4, in the MEMSdiffraction grating 51, currents flow through the outside parts 65 b and66 b of the coils 65 and 66 in the same direction. As a result, a largedriving force for swinging the movable portion 63 can be ensured.

[Function and Effect]

In the laser module 1, the MEMS diffraction grating 51 is disposed suchthat the normal line N of the diffraction grating portion 64 is inclinedwith respect to the first end surface 4 a of the QCL 4 and the length L1of the diffraction grating portion 64 in the first direction D1, whichis the arrangement direction of the lattice grooves 64 a, exceeds thelength L2 of the diffraction grating portion 64 in the second directionD2 perpendicular to the first direction D1. Since the diffractiongrating portion 64 is long in the first direction D1, the light from theQCL 4 can be satisfactorily received by the diffraction grating portion64 even when the diffraction grating portion 64 is disposed in aninclined manner. For example, it is possible to suppress a situation inwhich a part of the light from the QCL 4 becomes stray light withoutentering the diffraction grating portion 64. In addition, since thelength L2 of the diffraction grating portion 64 in the second directionD2 is reduced, the module can be reduced in size and it is possible tosuppress an increase in power consumption by suppressing an increase inthe size of the diffraction grating portion 64. Further, in the lasermodule 1, the MEMS diffraction grating 51 is disposed such that thefirst direction D1 is along the lamination direction (Z direction) ofthe laminated structure 42 when viewed in the Y direction perpendicularto the first end surface 4 a of the QCL 4. By disposing the MEMSdiffraction grating 51 such that the first direction D1 is along thelamination direction, the extension direction of the lattice groove 64 ain the diffraction grating portion 64 (second direction D2) can beorthogonal to the polarization direction of the light emitted from theQCL 4. In addition, by the first direction D1, which is the arrangementdirection of the lattice grooves 64 a, being the longitudinal directionof the MEMS diffraction grating 51, the lattice grooves 64 a positionedwithin the irradiation beam diameter in the inclined diffraction gratingportion 64 can be increased in number. As a result, a high wavelengthresolution of the MEMS diffraction grating 51 can be ensured and thespectral line width of the output light can be reduced. Accordingly,with the laser module 1, the size, power consumption, and spectral linewidth can be reduced. Ensuring the wavelength resolution of the MEMSdiffraction grating 51 and reducing the spectral line width tends to beparticularly important when a light source that emits light having amid-infrared long wavelength as in the case of the QCL 4 is used, andthe direction and number of lattice grooves are unlikely to be a problemwhen the wavelength is short as in the case of visible light.

The point that the wavelength resolution of the MEMS diffraction grating51 can be increased and the spectral line width can be reduced byincreasing the number of the lattice grooves 64 a will be described. Theindex (resolution) of how small Δλ can be distinguished as two spectrais represented by Equation (1) with regard to two spectral lines havingλ and λ+Δλ, which are two adjacent wavelengths.

λ/Δλ=mN×W   (1)

m represents the order of diffraction, and N×W represents the number ofthe lattice grooves 64 a.

A spectral line width of approximately 13 nm can be obtained when, forexample, the laser module 1 with an oscillation wavelength of 8000 nm isconfigured using the MEMS diffraction grating 51 with a period d of 7800nm as an actual measurement result. When Δλ is calculated in the case ofthe MEMS diffraction grating 51 with the period d of 7800 nm and thelength L1 of 4.2 mm, Δλ is 14.8 nm as a result of the following Equation(2) and a calculation result equivalent to the actually measuredspectral line width can be obtained.

Δλ=8000 nm/primary×(total number 538)=14.8 nm   (2)

It can be seen from the above that the wavelength resolution of thediffraction grating portion 64 can be increased and the spectral linewidth can be reduced by increasing the length L1 of the diffractiongrating portion 64.

The four corner portions 63 a of the movable portion 63 are formed in around shape. As a result, the moment of inertia of the movable portion63 can be reduced and the swing of the movable portion 63 can beincreased in speed.

The light emitted from the QCL 4 and incident on the MEMS diffractiongrating 51 via the lens 6 has a beam shape in which the length in thefirst direction D1 exceeds the length in the second direction D2 at theposition of the diffraction grating portion 64. Since the length L1 ofthe diffraction grating portion 64 in the first direction D1 exceeds thelength L2 of the diffraction grating portion 64 in the second directionD2, the light from the QCL 4 can be satisfactorily received by thediffraction grating portion 64 even in such a case.

The MEMS diffraction grating 51 is disposed such that the entire lightemitted from the QCL 4 and transmitted through the lens 6 is incident onthe diffraction grating portion 64 regardless of the swing angle of themovable portion 63 around the axis A. As a result, it is possible tosuppress a situation in which a part of the light from the QCL 4 becomesstray light without entering the diffraction grating portion 64.

[Modification Example]

The present disclosure is not limited to the above embodiment andmodification examples. For example, various materials and shapes notlimited to those described above can be adopted as the material andshape of each configuration. In the above embodiment, the pair of coils65 and 66 are configured by one coil wiring connected in series.Alternatively, the coils 65 and 66 may be configured separately andpulled out separately to the outside. From the viewpoint of currentincrease and controllability improvement, it is preferable that thecoils 65 and 66 are configured by one coil wiring. In the aboveembodiment, the MEMS diffraction grating 51 is driven by anelectromagnetic drive method. Alternatively, the MEMS diffractiongrating 51 may be driven by an electrostatic or piezoelectric method. Inthese cases, for example, electrostatic comb teeth or piezoelectricelements are provided in place of the coils 65 and 66, the magnet 52,and the yoke 53.

The shape of the movable portion 63 is not limited to a rectangularshape. For example, the shape may be an elliptical shape, a circularshape, or a square shape. The corner portion 63 a of the movable portion63 may not be formed in a round shape, and the corner portion may haveedge portions intersecting at right angles. The shape of the diffractiongrating portion 64 is not limited to a rectangular shape. The shape maybe, for example, an elliptical shape. The light emitted from the QCL 4and incident on the MEMS diffraction grating 51 via the lens 6 may havea substantially circular beam shape at the position of the diffractiongrating portion 64. The recess portion 56 may not be formed in themagnet 52.

The MEMS diffraction grating 51 may be configured as in a firstmodification example illustrated in FIG. 9A or a second modificationexample illustrated in FIG. 9B. Each connecting portion 62 extends in ameandering matter in the first and second modification examples. In thefirst modification example, the pair of connecting portions 62 aresymmetrically formed in relation to a straight line passing through thecenter of the movable portion 63 and perpendicular to the axis A in aplan view. In the second modification example, the pair of connectingportions 62 are asymmetrically formed in relation to the straight linein a plan view. With the first and second modification examples as wellas the above embodiment, the size, power consumption, and spectral linewidth can be reduced. In addition, the first and second modificationexamples can be suitably used when the movable portion 63 is operated ina non-resonant mode. When the movable portion 63 is operated in anon-resonant mode, the wavelength of the output light L can becontrolled and can be fixed to any wavelength.

What is claimed is:
 1. An external resonance-type laser modulecomprising: a quantum cascade laser including a laminated structure andemitting light from an end surface; a MEMS diffraction grating includinga movable portion capable of swinging around a predetermined axis and adiffraction grating portion formed on the movable portion, wherein theMEMS diffraction grating diffracts and reflects the light emitted fromthe quantum cascade laser by the diffraction grating portion, andreturns a part of the light to the quantum cascade laser; and a lensdisposed between the quantum cascade laser and the MEMS diffractiongrating, wherein the diffraction grating portion includes a plurality oflattice grooves arranged in a first direction and each of the pluralityof lattice grooves extends in a second direction perpendicular to thefirst direction, the MEMS diffraction grating is disposed such that anormal line of the diffraction grating portion is inclined with respectto the end surface and the first direction is along a laminationdirection of the laminated structure when viewed in a directionperpendicular to the end surface, and a length of the diffractiongrating portion in the first direction exceeds a length of thediffraction grating portion in the second direction.
 2. The externalresonance-type laser module according to claim 1, wherein the movableportion is formed in a shape having four corner portions when viewed ina direction parallel to the normal line, and the four corner portionsare formed in a round shape.
 3. The external resonance-type laser moduleaccording to claim 1, wherein the light emitted from the quantum cascadelaser and incident on the MEMS diffraction grating via the lens has abeam shape in which a length in the first direction exceeds a length inthe second direction at a position of the diffraction grating portion.4. The external resonance-type laser module according to claim 1,wherein the MEMS diffraction grating is disposed such that all of thelight emitted from the quantum cascade laser and transmitted through thelens is incident on the diffraction grating portion regardless of aswing angle of the movable portion around the axis.